Fix "prev_subject" with LEGACY_PREV_TOPIC.

I added `LEGACY_` prefix to the var, because
otherwise you'd have really confusing code
where you change the key from `PREV_TOPIC`
to "prev_topic".

zerver/lib/actions.py

@@ -61,7 +61,7 @@ from zerver.lib.topic import (
     save_message_for_edit_use_case,
     update_messages_for_topic_edit,
     ORIG_TOPIC,
-    PREV_TOPIC,
+    LEGACY_PREV_TOPIC,
     TOPIC_LINKS,
     TOPIC_NAME,
 )
@@ -3968,7 +3968,7 @@ def do_update_message(user_profile: UserProfile, message: Message, topic_name: O
         event[ORIG_TOPIC] = orig_topic_name
         event[TOPIC_NAME] = topic_name
         event[TOPIC_LINKS] = bugdown.topic_links(message.sender.realm_id, topic_name)
-        edit_history_event[PREV_TOPIC] = orig_topic_name
+        edit_history_event[LEGACY_PREV_TOPIC] = orig_topic_name
 
         if propagate_mode in ["change_later", "change_all"]:
             messages_list = update_messages_for_topic_edit(

zerver/lib/topic.py

@@ -23,9 +23,14 @@ from typing import Any, Dict, List, Tuple
 ORIG_TOPIC = "orig_subject"
 TOPIC_NAME = "subject"
 TOPIC_LINKS = "subject_links"
-PREV_TOPIC = "prev_subject"
 MATCH_TOPIC = "match_subject"
 
+# This constant is actually embedded into
+# the JSON data for message edit history,
+# so we'll always need to handle legacy data
+# unless we do a pretty tricky migration.
+LEGACY_PREV_TOPIC = "prev_subject"
+
 # This is used in low-level message functions in
 # zerver/lib/message.py, and it's not user facing.
 DB_TOPIC_NAME = "subject"

zerver/views/messages.py

@@ -38,6 +38,7 @@ from zerver.lib.topic import (
     topic_match_sa,
     user_message_exists_for_topic,
     DB_TOPIC_NAME,
+    LEGACY_PREV_TOPIC,
     MATCH_TOPIC,
 )
 from zerver.lib.topic_mutes import exclude_topic_mutes
@@ -1299,11 +1300,10 @@ def fill_edit_history_entries(message_history: List[Dict[str, Any]], message: Me
 
     for entry in message_history:
         entry['topic'] = prev_topic
-        if 'prev_subject' in entry:
-            # We replace use of 'subject' with 'topic' for downstream simplicity
-            prev_topic = entry['prev_subject']
+        if LEGACY_PREV_TOPIC in entry:
+            prev_topic = entry[LEGACY_PREV_TOPIC]
             entry['prev_topic'] = prev_topic
-            del entry['prev_subject']
+            del entry[LEGACY_PREV_TOPIC]
 
         entry['content'] = prev_content
         entry['rendered_content'] = prev_rendered_content